1. Field of the Invention
The invention relates to a control strategy for starting an engine in a hybrid electric vehicle powertrain having parallel power delivery path to vehicle traction wheels.
2. Background Art
In a hybrid electric vehicle powertrain of the type disclosed, for example, in U.S. Pat. No. 7,013,213 and in U.S. Patent Application Publication No. U.S. 2006/0016412 A1, an internal combustion engine and electric traction motor are used to develop vehicle traction wheel torque in a split power delivery path. The power delivery path is defined in part by a torque splitter planetary gear system in which a planetary carrier is drivably connected to an engine crankshaft and a ring gear is driveably connected through gearing to a differential-and-axle assembly for the vehicle traction wheels. An electric motor also is coupled to the differential-and-axle assembly through the gearing. The motor and an electric generator are electrically coupled, together with a battery, in a generator-motor-battery sub assembly. The generator is directly connected to a sun gear, which serves as a reaction member, as engine power is delivered to the gearing through the planetary gear unit.
Engine power output is divided into two parallel paths by controlling generator torque. A mechanical power flow path is established from the engine to the planetary gear unit and ultimately to a power output shaft. The other power flow path is an electrical power flow path that distributes power from the engine, to the generator, to the motor, and then to the power output shaft. The generator, the motor and the planetary gear unit thus may operate as an electricro-mechanical transmission with continuously variable ratio characteristics.
A vehicle system controller coordinates the divided power distribution. Under normal operating conditions, the vehicle system controller interprets a driver's demand for power as a function of acceleration or deceleration demand. It then determines when and how much torque each power source needs to provide in order to meet the driver's power demand and to achieve a specific vehicle performance while taking into consideration engine fuel economy, emission quality, etc. The vehicle system controller will determine the operating point of the engine torque and speed relationship.
The generator, when it is acting as a motor, can deliver power to the planetary gearing. That power can be used to provide engine cranking during an engine start. When the generator is acting as a generator, it is driven by the planetary gearing to achieved charging power for the battery. The generator can act as a generator when it is driven by the portion of the engine power that is not delivered mechanically through the transaxle gearing. The balance of the engine power delivered through the planetary gearing to the generator charges the battery and the battery drives the traction motor in a positive power split configuration. In this fashion, the two power sources, i.e., the engine and the generator-motor-battery subsystem, are integrated so that they work together seamlessly to meet a driver's demand for power. The system will achieve an optimum power split between the two power sources.
The generator acts as a starter motor for the internal combustion engine. The engine, during a normal operating cycle, must be started and stopped frequently. Each time it is started, it must be started quickly, quietly and smoothly over a large range of temperatures without violating battery power limits. The engine starting mode must not occur for an extended period of time in a so-called resonance zone during which engine torque delivery is unstable and characterized by torque spikes. A typical engine speed range for this so-called resonance zone would be approximately 300-500 rpm for a typical contemporary automotive vehicle engine.
Fulfillment of these various engine start requirements is difficult to achieve when the engine temperatures are very cold. A cold engine requires more energy for cranking because of increased friction of cold engine lubricants. Further, a cold battery cannot supply as much energy due to limitations of the chemistry of the battery. If the engine is designed with variable valve timing, the starting of a cold engine becomes even more difficult. That is because the addition of the variable intake valve timing feature for reduced noise vibration and harshness (NVH) of the engine reduces the pressures inside the engine cylinders. This in turn requires a higher cranking speed before the engine can start.
A cold start strategy is discussed in U.S. Patent Application Publication No. US 2006/0016412 A1. That cold start strategy requires a commanded target engine speed that is constant during a starting event. Variations in the torque required to crank the engine using the strategy of the '412 publication causes fluctuations in the power used to crank the engine. Thus, the transmission and engine friction in cold environments can cause over discharge of the high voltage battery. Further, a single strong transient combustion event in at least one of the engine cylinders can momentarily increase engine speed. The generator then is prompted by an increased engine speed signal to respond to the transient combustion event by reducing generator torque command. This can cause the generator to stop assisting the engine during the cranking mode, which can lead to engine stalls or a “no-start” situation because of an inherent control signal response time delay in the engine controller and because of a physical lag time caused by transient kinetic energy changes for the rotary mass of the crankshaft and components mechanically connected to the crankshaft.
If the engine successfully and consistently develops engine driving torque using a strategy of the kind described in the '412 patent application publication, engine speed is pulled through the resonance zone of approximately 300-500 rpm before the engine speed is increased to the desired engine idle speed. This can result in using more battery power than the battery can safely provide. This can result in low battery voltage situations as the battery's state of charge is depleted. This may cause a reduced battery life.